A coating is a covering that is applied to the surface of a substrate. Coatings can be decorative, functional, or both. An example of a decorative coating would be an artist's paint. An example of a functional coating would be an adhesive. An example of a coating with both a decorative and a functional purpose would be a house paint which would both protect the surface of the house and be aesthetically appealing.
Coatings are used in many applications, including paper, packaging, films, printing, construction products, automobiles, aircraft, marine products, and all sorts of other manufactured products. Furthermore, functional coatings can be applied to change the surface properties of the substrate, such as adhesion, wetability, corrosion resistance, or wear resistance. Aside from the decorative aspects, an important function of coatings is to protect the substrate from the environment and wear and tear under usage conditions.
Polyester containing coatings such as polyethylene terephthalate (PET) coatings are particularly useful. These coatings have been used in various forms for protective and barrier coatings, and other types of packaging, as well as for substrates themselves. The performance of polyester coatings has been postulated to be due to the inherent properties of the polyesters, such as high-strength, a partially aromatic backbone, and a variable degree of crystallinity. For example, the components of PET plastic are mainly terephthalic acid and ethylene glycol. These components also happen to serve as prime ingredients for many industrial paint resins for multiple substrates and applications.
Commonly, polyester polyols are made by condensing aromatic diacids, diesters, or anhydrides (e.g., terephthalic acid or dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. These starting materials usually derive exclusively from petrochemical sources.
However, it would be highly desirable to design high performance coatings made from a polyester polyol, wherein the polyol is derived from a polyacid source that is a recycled aromatic polyacid source. Furthermore, it would be highly desirable to incorporate polyols that incorporate other polymeric materials, particularly when these other polymeric materials are from recycled sources. However, there are challenges in developing such coatings based on these polyols to achieve robust performance characteristics such as resistance to corrosion and abrasion. As companies increasingly seek to offer products with improved sustainability, the availability of intermediates produced from bio-renewable and/or recycled materials becomes more leveraging. However, there remains a need for these products to deliver equal or better performance than their traditional petroleum-based alternatives at a comparable price point.
Depending on the substrate being protected, and the environment it is being protected from, recycled PET (rPET) can be a starting point from which to design high performance protective coatings with long service lifetimes. Starting from a water bottle on a supermarket shelf, the transition to such a protective coating would be an example of “upcycling”, as opposed to a straight recycle category or the more commonly practiced “downcycling”. Most plastic recycling is classified as “downcycling”, meaning it reduces the quality of a material over time. See McDonough, W., and Branugart, M., Cradle to Cradle: Remaking the Way We Make Things, 2002, North Point Press, p. 56-57 (2002), which is incorporated by reference herein in its entirety.
Polyols suitable for use in making high-quality polyurethanes have proven difficult to manufacture from recycled materials, including recycled polyethylene terephthalate (rPET). Many references describe digestion of rPET with glycols (also called “glycolysis”), usually in the presence of a catalyst such as zinc or titanium. Digestion converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers. Although such mixtures have desirably low viscosities, they often have high hydroxyl numbers or high levels of free glycols. Frequently, the target product is a purified bis(hydroxyalkyl) terephthalate (see, e.g., U.S. Pat. Nos. 6,630,601, 6,642,350, and 7,192,988) or terephthalic acid (see, e.g., U.S. Pat. No. 5,502,247), which are incorporated by reference herein in their entirety. Some of the efforts to use glycolysis product mixtures for urethane manufacture are described in a review article by D. Paszun and T. Spychaj (Ind. Eng. Chem. Res. 36 (1997) 1373, which is incorporated by reference herein in its entirety.
Frequently, ethylene glycol is used as the glycol reactant for glycolysis, because it minimizes the possible reaction products. Usually, the glycolysis is performed under conditions effective to generate bis(hydroxyethyl) terephthalate (“BHET”), although sometimes the goal is to recover pure terephthalic acid. When ethylene glycol is used as a reactant, the glycolysis product is typically a crystalline or waxy solid at room temperature. Such materials are less than ideal for use as polyol intermediates because they must be processed at elevated temperatures. Polyols are desirably free-flowing liquids at or close to room temperature. The challenges for these polyols and their incorporation into other products become even more severe when additional polymeric components, particularly other recycled polymeric components, are used to prepare the polyols.
It has surprisingly been found in the present invention that polyols containing components derived from bisphenols and bisphenol polymers, such as poly(bisphenol-A carbonate) (PBAC), particularly recycled poly(bisphenol-A carbonate) (rPBAC) can be used to prepare coatings having desirable performance characteristics.
It is apparent from the above there is an ongoing need for sustainable sources of polyester polyols which at the same time can help to both reduce waste streams, and to provide further options for using under-utilized recycled polymer streams and incorporating them into products such as high performance coatings.